This invention relates to a video disk and its recorder for recording television video signals.
Disks for dense data recording have recently been developed and commercialized. Video disks and digital audio disks are typical of these dense recording disks. In the case of an optical video through the frequency modulation of a carrier wave at 8.1 MHz the sync chip level and the white level may respectively be at 7.6 MHz and 9.3 MHz, whereas accompanying audio signals such as bilateral stereo and bilingual signals are recorded through the frequency modulation of carrier waves at 2.3 MHz and 2.8 MHz. On the other hand, the spectrum of the EFM signal of a PCM bilateral stereo audio signal in an optical digital audio disk is seen to occupy frequency bands at about 2 MHz or lower. Since the frequency bands at 2 MHz or lower are mostly vacant, such an EFM signal can be recorded on video disks through multiplex frequency division.
In such video and digital audio disks, time codes (representing MIN, SEC, FRAME, etc.) as an absolute address are recorded on a frame (block) basis. In the case of, for instance, the video disk, the time code is incremented by one second every 30 frames from the beginning of a program because the frequency of the frame is 29.97 Hz (for the NTSC system) and frame numbers are allotted to the frames numbered 0 through 29 within the same second. In other words, as shown in FIG. 4(a), the frame 10 sec 29 is followed by the frame 11 sec 0 and then the frame 11 sec 1.
In the digital audio disk, time codes are recorded on a 75 Hz block (frame) basis. In this case, 75 blocks become equal to one second and therefore the time code is incremented by one second every 75 blocks and frame numbers 0 through 74 are further allotted to the blocks (frames) within the same second. In other words, as shown in FIG. 4(b), the frame 10 sec 74 is followed by the frame 11 sec 0 and then the frame 11 sec 1.
Since the frequency of the block of the digital data in a audio disk is exactly 75 Hz, the time code coincides with real time. In other words, precisely one second elapses when data of 75 blocks are played back. On the contrary, the time code in a video disk will not precisely coincide with real time because the frequency of the frame of the video signal is 29.97 Hz, i.e., an error of about 3.6 seconds will be produced per hour of real time. As such an error is insignificant, when only a video signal is recorded on the video disk, there is no particular problem even though the time code of the video signal is used for retrieval. However, if a digital data signal superposed on the video signal is continuously recorded on the video disk and each of the two signals is recorded with its own time code, the position retrieved from the time code of the digital data signal will differ from what is retrieved from the time code of the video signal, thus impeding its practicality.